The proposed research is designed to investigate the physiological effects of thoracic inferior vena cava constriction in dogs during the acute phase and for a period of time post-constriction (usually 4-6 weeks) when most of these animals appear to reach a relatively stable state with less avid sodium retention. The studies will combine continuous measurements of sodium and water balance with a series of acute clearance experiments performed during a water diuresis at prescribed intervals in which plasma renin activity, blood volume, systemic and renal hemodynamics, intrarenal blood flow distribution assessed by Xenon-133 washout and renal sodium and water transport will be examined. In addition, the response of this animal model of sodium and water retention to angiotensin II and its antagonist and alpha-adrenergic blockade will be assessed serially to provide information regarding the physiological role of the renin-angiotensin-aldosterone and adrenergic systems at various stages. This research is proposed in an attempt to examine the hypothesis that the dog with thoracic inferior vena cava constriction is a dynamic model with changing physiological alterations and that early sodium and water retention which leads to extracellular fluid volume expansion is a major determinant of the course of events. Correlations will be made between cumulative sodium and water balance and any physiological or pharmacological response change noted in these dogs over the time span examined.